1. Field of the Invention
The present invention relates to a lubricant surface-treated pipe for hydroforming use having excellent lubrication for hydroforming.
2. Description of the Related Art
Hydroforming of a steel pipe is known as a typical method for forming T-shaped pieces (hereinafter referred to as T pieces) to be used at branches of piping.
Drawings will be referred to for explaining the general structure of T pieces and conventionally-recognized problems.
FIGS. 1A and 1B show a T piece, wherein FIG. 1A shows a side view and FIG. 1B shows a sectional view taken along line A--A of FIG. 1A. A T piece 1 includes a trunk 1a and a branch 1b, having a height H, which merge together through a crotch 1c, having a smooth curvature (radius R).
FIGS. 2A and 2B show dies used for hydroforming a T piece, wherein FIG. 2A shows a longitudinal sectional view and FIG. 2B shows a side view. Dies 2 include an upper die 2a and a lower die 2b which are vertically separable from each other.
A semicircular groove 2a-1 is formed in the upper die 2a, whereas a semicircular groove 2b-1 and a hole 2b-2 are formed in the lower die 2b. The shape of the thus-formed die cavity is identical to the profile of the T piece 1. The dies 2 are generally of a tool steel. The surface of the die cavity is smoothly finished and is hardened through heat treatment or chromium-plating.
FIGS. 3A to 3D show the steps of hydroforming a T piece.
In a step shown in FIG. 3A, a tubular blank 3, having a predetermined length L.sub.0 is set in the dies 2 of a hydroforming machine (not shown). An outer diameter D1 and a wall thickness t.sub.a of the tubular blank 3 are identical to those of a product T piece. A tubular blank is a short piece of pipe obtained by cutting a long steel pipe into pieces, each having such a length as to be accommodated in the hydroforming dies. The tubular blank 3 is set in the groove 2b-1 of the lower die 2b, and then the upper die 2a attached to the vertically pressing apparatus of a hydroforming machine (not shown) is lowered, thereby setting the tubular blank 3 in the dies 2. In order to hold the upper die 2a in position during hydroforming, the upper die 2a is pressed against the lower die 2b by a predetermined force.
In a step shown in FIG. 3B, opposed pushing blocks 4 and 5 attached to the horizontally pressing apparatus of a hydroforming machine (not shown) are advanced to press the end surfaces 4a and 5a thereof against the end surfaces 3a of the tubular blank 3. Then, the tubular blank 3 is filled with a hydraulic fluid 8 injected through a hydraulic fluid path 6. The hydraulic fluid is usually of an emulsion prepared through combination of water with oil, which oil is intended primarily for rust prevention. Subsequently, while the pressure of the hydraulic fluid 8 is increased, the pushing blocks land 5 are advanced. As a result, the tubular blank 3 begins to deform along the round (R) portion 2b-3 of the hole 2b-2 of the lower die 2b, so that part of the tubular blank 3 begins to project into the hole 2b-2.
In a step shown in FIG. 3C, the tubular blank 3 is contracted to a length L' slightly longer than the length of the trunk of a product T piece, and a projected portion 9b is formed against the hole 2b-2 and a stopper 7 set in a predetermined position. Thereafter, the pressure of the hydraulic fluid 8 is reduced, the pushing blocks 4 and 5 are retreated, and the upper die 2a is raised. The stopper 7 is raised by a cylinder (not shown) to thereby eject a semifinished product 9 from the lower die 2b.
FIG. 3D shows a side view of the semifinished product 9. The projected portion 9b is cut at a height H, and a trunk 9a is finished to a length L, followed by heat treatment, as needed, to thereby obtain a T piece.
In the steps shown in FIGS. 3A to 3C, the hydraulic fluid 8 applies to the tubular blank 3 a pressure ranging from hundreds of atmospheres to one thousand and several hundreds of atmospheres. In addition, the pushing blocks 4 and 5 apply a compressive force to the tubular blank 3. Accordingly, a high pressure acts on the outer surface of the tubular blank 3 and the groove 2a-1 of the upper die 2a and the groove 2b-1 of the lower die 2b.
Also, a high pressure acts on the R portion 2b-3 of the hole 2b-2 of the lower die 2b along which the work-hardened tubular blank 3 slides. Under these circumstances, the following problems anise in association with the friction induced between the tubular blank 3 and the die surface when the tubular blank 3 is compressed in an axial direction in the die grooves 2a-1 and 2b-1, or when part of the tubular blank 3 projects into the die hole 2b-2.
First, scratches are formed in the outer surface of the semifinished product 9, and these must be removed by polishing with a grinder or the like. A mentioned previously, the cavity of the dies 2 is finished hard and smooth. However, since hydroforming involves severe friction, repeated hydroforming results in the formation of scratches even in the dies 2. Correction of the die surface through polishing reduces production efficiency, and repeated correction results in a change of dimensions of a product. In such a case, the corrected portion of the die surface must be padded and finished, resulting in an increase in maintenance cost.
Second, since the tubular blank 3 is difficult to slide in an axial direction, the tubular blank 3 is likely to buckle in the vicinity of end portions thereof. Thus, hydroforming a thin-walled product is difficult.
FIG. 4 shows a buckled tubular blank 3. As shown in FIG. 4, buckling 10 is likely to occur in the vicinity of end portions of the tubular blank 3.
Third, since part of the tubular blank 3 becomes difficult to project into the die hole 2b-2, the projected portion 9b is likely to crack.
FIG. 5 shows the cracked projected portion 9b. As shown in FIG. 5, a crack 11 occurs in the top area of the projected portion 9b when the tubular blank 3 becomes difficult to project into the die hole 2b-2.
In solving the above problems involved in hydroforming, it is important to reduce frictional resistance involved in sliding motion between a tubular blank and dies under a very high surface pressure.
In order to reduce such frictional resistance, the outer surface of a tubular blank is treated against galling to the die surface. A lubrication oil may be applied onto the outer surface of a tubular blank, but this method is relatively ineffective because the lubrication oil is rubbed off due to sliding under a high surface pressure between the tubular blank and the dies. Also, a water-based hydraulic fluid used for applying an internal pressure to a tubular blank accommodated in dies may deteriorate the effect of a lubrication oil.
Accordingly, anti-galling paint is commonly employed. A tubular blank obtained by cutting a pipe to a predetermined length is degreased and then paint is applied on the outer surface thereof by spraying or brushing. After the applied paint is sufficiently dried and solidified, hydroforming is performed.
However, this method involves the following problems.
First, the degreasing and painting of a tubular blank require corresponding labor or man-hours. Since painting a long pipe it is difficult, the pipe is cut into tubular blanks, each having a predetermined length, and then each of the tubular blanks is painted. In this case, the step of cutting a pipe into tubular blanks cannot be continuously linked to a hydroforming step. Accordingly, material stagnates between steps, requiring an excess space for storing material and impairing the overall efficiency of a hydroforming system.
Second, since each of the tubular blanks, which are cut into a predetermined length from a long steel pipe, is painted by hand, such painting requires not only painting time, but also skill to paint the curved surface of a tubular blank to a uniform thickness. In the case of a thin-walled tubular blank, if the painting thickness is nonuniform, the tubular blank will be highly likely to buckle at a portion where the painting thickness changes, while being compressed in a longitudinal direction thereof during hydroforming. When the coating of paint is excessively thick, the coating of paint will adhere to the die surface. Such adhering paint will dimple on the surface of a product in the next hydroforming process. Accordingly, upon completion of hydroforming, such adhering paint must be removed before the next hydroforming process starts, thus wasting time and labor.
Third, in painting, paint is likely to adhere thick to part of the end surface of a tubular blank. This may disable sealing at the end surface of a tubular blank when a hydraulic fluid is injected into the tubular blank for hydroforming as shown in FIG. 3C. Accordingly, before hydroforming is started, the end surfaces of a tubular blank must be visually checked, and any adhering paint must be removed therefrom.
Fourth, when the coating of paint must be removed through use of an organic solvent after hydroforming is completed, much labor and time is required. Also, a problem in working environment may arise.